[keycloak-user] Curious about the Email constraint on UserEntity

I have been testing Keycloak (1.1.0 Final) federation ability against some of our existing user databases. I came across the unique email address constraint in the UserEntity table. What is the reasoning behind email addresses being unique? Our particular use case is that a user can create multiple usernames and have the same email address across many of them if they choose to. Adil Arif